Second generation mobile communication, including CDMA and GSM, means transmission and reception of voice through digital data. As an advance from the GSM, GPRS has been proposed, which is a technology for providing a packet switched data service based on the GSM system.
Third generation mobile communication means communication of image and data as well as voice, and 3GPP (Third Generation Partnership Project) developed mobile communication system (IMT-2000) technology and adopted WCDMA as the Radio Access Technology (RAT). The IMT-2000 technology and Radio Access Technology (RAT), i.e. WCDMA, are comprehensively referred to as UMTS (Universal Mobile Telecommunication System) in Europe. And, UTRAN stands for UMTS Terrestrial Radio Access Network.
Now the Third generation mobile communication is evolving into Fourth generation mobile communication.
As a Fourth generation mobile communication technology, two methods have been proposed, one being Long-Term Evolution Network (LTE) technology which has been under development for standardization by 3GPP and another being IEEE 802.16 which has been under development for standardization by IEEE. For LTE, the term E-UTRAN (Evolved-UTRAN) is being used.
In the Fourth generation mobile communication technology, OFDM (Orthogonal Frequency Division Multiplexing)/OFDMA (Orthogonal Frequency Division Multiple Access) has been introduced. OFDM uses multiple orthogonal subcarriers. OFDM uses orthogonality characteristics between IFFT (inverse fast Fourier Transform) and FFT (fast Fourier Transform). The transmitter transmits data by performing data IFFT to the data. The receiver restores original data by performing FFT to the data received. The transmitter uses IFFT for combining multiple subcarriers and the receiver uses corresponding FFT for separating the multiple subcarrier.
Meanwhile, many efforts are being made in Third or Fourth generation mobile communication system to increase the capacities of cells to support high bandwidths and bidirectional services such as multimedia contents and streaming.
Also, there have been approaches to use higher frequency bandwidths for increasing cell capacities and reduce cell radii. By using cells with small radius such as pico cell, it is possible to use higher bandwidth than the frequencies used in conventional cellular system, thereby providing the merit of transmitting more information. These method, however, have disadvantage of more cost for installing more base stations in the same area.
In the efforts to increase the capacities of cells, the methods of using femtocell have been proposed recently.
Femtocell means providing small scale wireless environment by installing compact base stations which consume low power at indoor space of home or office. The femtocell can enhance the quality of service by extending the area where the service is available and improving the capacities of the service, and is expected to stabilize next generation mobile communication system by providing data service.
Meanwhile, recent communication terminal devices are generally equipped with Wi-Fi module or Bluetooth. Especially, as smart phones are widely used, it is common that various heterogeneous radio access technologies such as cellular (Third generation or Fourth generation communication) access technology, WLAN, Bluetooth and ZigBee are equipped in a handset. These types of devices equipped with various heterogeneous radio access technologies in a handset is called dual mode handsets.
As the dual mode handsets described above is widely used, the technology of FMC (Fixed Mobile Convergence) is newly being proposed.
According to the FMC technology, when the user with a handset of the technology approaches a UMA (Unlicensed Mobile Access) based Access Point (or wireless base station), the service identical with the Third generation or Fourth generation communication technology is provided by using unlicensed wireless frequencies.
FIG. 1 illustrates the technology of FMC.
As can be seen by referring to FIG. 1, terminal 10 and access point 20 are within the coverage of the base station 30. The access point 20 can use Wireless LAN or Bluetooth which are based on Wi-Fi.
When the terminal 10, which is under communication with the base station 30, enters the coverage of the access point 10 using Wi-Fi or Bluetooth, the base station 30 performs a handover to the access point 20 so that the communication can be performed seamlessly.
Recently, various methods are being developed including the method of communication in which a user can communicate by using femto base station, Third generation or Fourth generation cellular communication technology at personal space such as home or office by combining the FMC communication method and femto base station method, and also can communicate, as needed, by using unlicensed bands such as Wi-Fi.
In order to use the FMC technology, the terminal should search for neighboring access points. Periodic search of access points, however, requires much power consumption and the resources of the terminal are also consumed, thereby affecting the cellular mobile communication under current connection.